Paleogeography as a climatic forcing factor     

Eric J. Barron 《International Journal of Earth Sciences》1981,70(2):737-747

The cause of warm, equable ice free climates which dominate Phanerozoic earth history and the cause of the transition to glacial epochs are fundamental problems in paleoclimatology. Based on several criteria, paleogeography is one of the most likely mechanisms of climatic change on this time scale. Paleogeography modifies climate by inducing changes in atmospheric and oceanic circulation patterns, and by modifying the global albedo because of the contrasts in the albedo of different surfaces (e. g. ocean, land and snow) and because incoming solar radiation is a strong function of latitude. Using both simple and state-of-the-art planetary albedo models, the importance of this second hypothesis is tested as a mechanism of climatic change. It is concluded that paleogeography, by directly altering the planetary albedo, operates in the correct sense to explain the Tertiary global cooling and close to the correct order of magnitude to explain the contrast between warm, equable Mesozoic climates and the present glacial climates.

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Zusammenfassung Die Ursache des warmen, ausgewogen eisfreien Klimas, das in phanerozoischer Erdgeschichte dominiert, und die Ursache des Übergangs zu glazialen Epochen sind fundamentale Probleme in der Paläoklimatologie. Aufgrund mehrerer Kriterien wird Paläogeographie als einer der wahrscheinlichsten Mechanismen für Klimaänderungen auf dieser Zeitskala angesehen. Paläogeographie verändert das Klima durch Änderungen atmosphärischer und ozeanischer Zirkulationssysteme, durch Modifizierung der weltweiten Albedoverteilung auf Grund unterschiedlicher Albedo über verschiedenen Oberflächen (Ozean, Land, Schnee) sowie durch die starke Abhängigkeit der einfallenden Sonneneinstrahlung von der geographischen Breite.Die Benutzung einfacher und — beim heutigen Stand der Theorie — fortgeschrittener Modelle erlaubt eine Überprüfung der Bedeutung dieser zweiten Hypothese für klimatische Änderungen. Es wird gefolgert, daß Paläogeographie, durch die direkte Modifizierung planetarischer Albedo, den globalen Kühlungstrend im Tertiär vorzeichenrichtig andeutet und größenordnungsmäßig den Unterschied zwischen dem warmen, ausgewogenen mesozoischen Klima und dem derzeitigen glazialen Klima erklärt.

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Résumé La cause de l'existence de climates chauds, dépourvus de fluctuations extrêmes et de glaciations, qui ont dominé l'histoire de la Terre durant le Phanérozoique et la cause de la transition à des périodes glaciaires posent des problèmes fondamentaux à la paléoclimatologie. Basée sur plusieurs évidences, la paléogéographie est un des méchanismes les plus probables qui soit à l'origine des changements climatiques considérés sur cette échelle de temps. La paléogéographie introduit des changements de climat en induisant des perturbations dans les modèles de circulation atmosphérique et océanique et en modifiant l'albedo global à cause des contrastes dans l'albedo des différentes surfaces (ex: océan, continent et neige), et parce que la radiation solaire reçue est en étroite relation avec la latitude. A partir de deux modèles, l'un simple et le second le plus éllaboré à l'heure actuelle, l'importance de la seconde hypothèse est testée comme un méchanisme de changement climatique. On arrive à la conclusion que la paléogéographie, en modifiant d'emblée l'albedo à l'échelle du globe, joue dans le bon sens pour expliquer le refroidissement général durant le Tertiaire, et est très voisin de l'ordre de grandeur correct pour expliquer l'opposition entre les climats chauds, dépourvus d'extrêmes fluctuations durant le Mésozoique, et les climats glaciaires d'aujourd'hui.

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Platinum mineralization in the Alaskan-type intrusive complexes near Fifield,N.S.W., Australia. Part 2. Platinum-group minerals in placer deposits at Fifield     

Dr. E. Slansky  Dr. L. M. Barron  Mr. D. Suppel  Dr. Z. Johan  Dr. M. Ohnenstetter 《Mineralogy and Petrology》1991,43(3):161-180

Summary The Fiîeld Platinum Province, New South Wales, Australia, contains Alaskan-type mafic-ultramafic complexes with notable PGE mineralization. This mineralization has evolved in several stages ranging from high- to low-temperature hydromagmatic, with weathering and latertic concentration followed by erosion and placer accumulation in alluvial channels. This last stage has furnished the largest quantity of platinum from the area. PGM nuggets were collected in a corridor, south and southeast of Fifield, joining the leads exploited in the past and continuing northwards.The following PGM association was observed in the nuggets: isoferroplatinum, native osmium, osmiridium, iridosmine, laurite and bowieite. Isoferroplatinum is the matrix mineral in the nuggets, while (It, Os, Pt) alloys occur as exsolutions. The nuggets sometimes carry inclusions of euhedral chromite. Two sulphidic phases-laurite and bowieite-are rare and occur embedded as trapped early formed crystals in isoferroplatinum.This PGM association is entirely different, when compared to the PGM in pegmatoid clinopyroxenites (P-units) described previously (Johan et al., 1989), both as to the nature of the mineral species and to the chemistry of isoferroplatinum common to both occurrences. This proves conclusively that the source for the alluvial nuggets must be of a different nature from the presently established hard rock mineralization.The co-existence of iridosmine and osmiridium exsolutions in isoferroplatinum proves a high temperature origin for the nuggets, and excludes a low temperature origin through in situ overgrowth from aqueous solutions in either the laterite or the erosive profile. The presence of chromite inclusions of a certain diagnostic composition indicates that the original host rock for the nuggets was a chromitite within dunite. The chemistry of the coexisting alloy phases provides a temperature estimate of about 800 °C, which is compatible with Alaskan complexes in Alaska and the Urals. The textural study of the nuggets shows different rates of cooling from nugget to nugget, with many textures relating to heterogeneous nucleation in metallurgical processes.

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Platin-Mineralisation in Intrusiv-Komplexen des Alaska-Types bei Fifield, NSW, Australien. Teil 2. Platingruppen-Minerale in Seifenlagerstätten in Fifield

Zusammenfassung Die Platin-Provinz von Fifeld, New South Wales, Australien, enthält mafischultramafische Komplexe des Alaska-Typs mit beachtlicher PGE Mineralisation. Diese hat sich in verschiedenen Stadien entwickelt, die von hoch- bis tief- temperiert hydromagmatisch reichen, und von Verwitterungsbildungen, lateritischer Konzentration, Erosion und Seifenbildung in alluvialen Rinnen gefolgt wurde. Dieses letzte Stadium hat bisher die größte Menge an Platin produziert. PGM Nuggets wurden in einem Korridor südlich und südöstlich von Fifield gesammelt, und diese setzten sich in den Anreicherungen fort, die in der Vergangenheit ausgebeutet wurden.Die folgende PGM Paragenese wurde in den Nuggets beobachtet: Isoferroplatin, gediegen Osmium, Osmiridium, Iridosmin, Laurit und Bowieit. Isoferroplatin ist das Matrix-Mineral der Nuggets, während (Ir, Os, Pt)—Legierungen als Entmischungen erscheinen. Die Nuggets enthalten gelegentlich Einschlüsse von isomorphem Chromit. Zwei Sulfidphasen—Laurit und Bowieit-sind selten und kommen als früh gebildete Kristalle im Isoferroplatin vor.Diese PGM-Paragenese unterscheidet sich grundsätzlich von der in pegmatoiden Klinopyroxeniten (P-units;Johan et al., 1989). Dies betrifft sowohl die Art der Minerale wie auch die chemische Zusammensetzung von Isoferroplatin. Diese Beobachtungen zeigen in überzeugender Weise, daß die Quelle für die alluvialen Nuggets nicht in der jetzt nachgewiesenen Vererzung in Festgesteinen gesucht werden kann.Die Koexistenz von Iridosmin- und Osmiridium-Entmischungen in Isoferroplatin weist auf die hohe Bildungstemperatur der Nuggets hin und schließt eine Tieftemperatur-Bildung durch in situ Überwachsung aus wässerigen Lösungen entweder im Laterit oder im Erosionsprofil aus. Die Anwesenheit von Chromiteinschlüssen definierter Zusammensetzung zeigt, daß das ursprüngliche Muttergestein der Nuggets ein Chromitit innerhalb von Duniten war. Die chemische Zusammensetzung der koexistierenden Legierungen weist auf Temperaturen von ungefähr 800°C; das ist in guter übereinstimmung mit Daten von den Komplexen in Alaska und im Ural. Texturelle Untersuchungen lassen Abkühlungsraten erkennen, die von Nugget zu Nugget schwanken; viele der beobachteten Texturen lassen sich mit heterogener Nukleation in metallurgischen Prozessen vergleichen.

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With 3 plates and 4 figures  相似文献   

Contrasting patterns in the abundance of fish communities targeted by fishers on two coral reefs in southern Mozambique     

T Sancelme  J Goetze  S Jaquemet  MG Meekan  A Flam  AM Watts 《African Journal of Marine Science》2020,42(1):95-107

Coastal populations of maritime countries in eastern Africa rely on fish as a primary source of protein, but baseline information on the abundance of fish communities on these coastlines is often lacking. We used baited remote underwater video stations to compare the abundance and diversity of reef fishes targeted by fishing at two sites in southern Mozambique, one at Lighthouse Reef within the Bazaruto Archipelago National Park and the other to the south at San Sebastian Reef on the San Sebastian Peninsula. Fish that are known targets of fisheries (mostly small-scale and artisanal) had an abundance that was almost three-times greater at San Sebastian Reef (80.22 ind. h^–1 [SE 18.00]) than at Lighthouse Reef (29.70 ind. h^–1 [SE 8.91]). Similarly, there was greater mean species richness at San Sebastian Reef (38.74 species h^–1 [SE 2.79]) than at Lighthouse Reef (25.37 species h^–1 [SE 3.66]). The main drivers of targeted fish abundance were habitat and depth, with shallow (<15 m) and mixed reef areas having the greatest abundance and richness. More sampling was done over sand habitat at Lighthouse Reef, which likely led to the lower abundance and species richness observed at this site; however, that finding could also be attributable to the fact that protection is provided to only a section of available coral reef habitat in a small area. Nevertheless, fish community structure was comparable between the sites, with similar proportions of carnivores (78–81%), herbivores (12–14%) and omnivores (7–8%). Our findings highlight the variation in species abundance and assemblages of coral-reef fish targeted by fishing in Mozambique and emphasise the importance of localised environmental variables as a driver of these patterns. To ensure maximum protection of Lighthouse Reef fish communities, we recommend an extension of the no-take zone to include the entire reef complex.  相似文献   

Validation of interannual simulations from the 1/8° global Navy Coastal Ocean Model (NCOM)     

A. Birol Kara  Charlie N. Barron  Paul J. Martin  Lucy F. Smedstad  Robert C. Rhodes 《Ocean Modelling》2006,11(3-4):376-398

A 1/8° global version of the Navy Coastal Ocean Model (NCOM) is used for simulation of upper-ocean quantities on interannual time scales. The model spans the global ocean from 80°S to a complete Arctic cap, and includes 19 terrain-following σ- and 21 fixed z-levels. The global NCOM assimilates three-dimensional (3D) temperature and salinity fields produced by the Modular Ocean Data Assimilation System (MODAS) which generates synthetic temperature and salinity profiles based on ocean surface observations. Model-data intercomparisons are performed to measure the effectiveness of NCOM in predicting upper-ocean quantities such as sea surface temperature (SST), sea surface salinity (SSS) and mixed layer depth (MLD). Subsurface temperature and salinity are evaluated as well. An extensive set of buoy observations is used for this validation. Where possible, the model validation is performed between year-long time series obtained from the model and time series from the buoys. The statistical analyses include the calculation of dimensionless skill scores (SS), which are positive if statistical skill is shown and equal to one for perfect SST simulations. Model SST comparisons with year-long SST time series from all 83 buoys give a median SS value of 0.82. Model subsurface temperature comparisons with the year-long subsurface temperature time series from 24 buoys showed that the model is able to predict temperatures down to 500 m reasonably well, with positive SS values ranging from 0.18 to 0.97. Intercomparisons of MLD reveal that the model MLD is usually shallower than the buoy MLD by an average of about 15 m. Annual mean SSS and subsurface salinity biases between the model and buoy values are small. A comparison of SST between NCOM and a satellite-based Pathfinder data set demonstrates that the model has a root-mean-square (RMS) SST difference of 0.61 °C over the global ocean. Spatial variations of kinetic energy fields from NCOM show agree with historical observations. Based on these results, it is concluded that the global NCOM presented in this paper is able to predict upper-ocean quantities with reasonable accuracy for both coastal and open ocean locations.  相似文献   

Seasonal rainfall partitioning under runon and runoff conditions on sandy soil in Niger. On-farm measurements and water balance modelling     

J. Rockstrm  P-E. Jansson  J. Barron 《Journal of Hydrology》1998,210(1-4):68-92

In sparsely cropped farming systems in semi-arid tropics, rainfall partitioning can be complex due to various interactions between vertical and horizontal water flows, both in the atmosphere and in the soil. Despite this, quantifying the seasonal rainfall partitioning is essential, in order to identify options for increased yields. Results are presented on water flow components, based on field measurements and water balance modelling, for three years (1994–96) in a farmer's field cultivated with pearl millet [Pennisetum glaucum (L.) Br.] in the Sahel (Niger). Water balance modelling was carried out for three common infiltration categories: runoff producing surfaces, surfaces receiving inflow of runon water from upstream zones, and a reference surface with zero runoff and runon. Runoff was calculated to 25%–30% of annual rainfall (which ranged from 488 to 596 mm), from crust observations, rainfall, soil wetness data, and infiltration estimates. Inflow of runon was estimated from field observations to 8%–18% of annual rainfall. The parameters in the functions for soil surface and canopy resistances were calibrated with field measurements of soil evaporation, stomatal conductance and leaf area. The model estimates of soil water contents, which were validated against neutron probe measurements, showed a reasonable agreement with observed data, with a root mean square error (RMSE) of approximately 0.02 m³ m⁻³ for 0–160 cm soil depth. Estimated productive water flow as plant transpiration was low, amounting to 4%–9% of the available water for the non-fertilised crop and 7%–24% for the fertilised crop. Soil evaporation accounted for 31%–50% of the available water, and showed a low variation for the observed range of leaf area (LAI <1 m² m⁻²). Deep percolation was high, amounting to 200–330 mm for the non-crusted surfaces, which exceeded soil evaporation losses, for 1994–95 with relatively high annual rainfall (517–596 mm). Even a year with lower rainfall (488 mm) and a distinct dry spell during flowering (1996), resulted in an estimated deep percolation of 160 mm for the non-fertilised crop. The crop did not benefit from the additional inflow of runon water, which was partitioned between soil water storage and deep percolation. The only exception to this was the fertilised crop in 1996, where runon somewhat compensated for the limited rainfall and the higher water demand as a result of a larger leaf area than the non-fertilised crop. The effects of rainfall erraticness, resulting in episodic droughts, explain why a crop that uses such a small proportion of the available water, in an environment with substantial deep percolation, still suffers from water scarcity. Application of small levels of phosphorus and nitrogen roughly doubled yields, from 380 to 620 kg ha⁻¹, and plant transpiration, from 33 to 78 mm. Evapotranspirational water use efficiency (WUE_ET) was low, 6500–8300 m³ ton⁻¹ grain for non-fertilised crop, which is an effect of the low on-farm yields and high non-productive water losses. The estimated seasonal rainfall partitioning indicates the possibility of quantifying vertical water flows in on-farm environments in the Sahel, despite the presence of surface overland flow.  相似文献   

An algorithm for the computer arrangement of reactions about invariant points     

Lawrence M. Barron  B. Jane Barron 《Contributions to Mineralogy and Petrology》1977,62(1):103-107

A computerised algorithm is used to arrange fluid-absent reactions about invariant points in P—T space for an end-member model of blueschist facies metamorphism at Port Macquarie, N.S.W., consisting of the nine phases quartz, albite, jadeite, lawsonite, zoisite, paragonite, glaucophane, pyrope and chlorite. Inspection of the print-out (a table of reaction take-off angles for each invariant point) indicates that this multisystem consists of two mutually exclusive sets of invariant points; lawsonite-absent, paragonite-absent and glaucophane-absent versus the other six. The algorithm is completely general for any two intensive variables and can treat solid solution minerals or degeneracy in reactions/invariant points.Terms ABS(X) Absolute value of X - A(J) Slope of reaction J in degrees - V(J) Denominator variable in the slope fraction for reaction J - R(K, J) Reaction coefficient of phase K in reaction J - S(K, J) Stability pointer for phase K in reaction J, in degrees - SGN (X) Library function: =–1 when X<0; =+1 when X0 - B(Q) Correct take-off angle for the Q-absent reaction in degrees  相似文献   

Some thermodynamic properties of the Berman and Brown model for CaO-Al₂O₃-SiO₂     

Lawrence M. Barron 《Geochimica et cosmochimica acta》1986,50(12):2727-2733

The and (1984) excess free energy model (B&B) is extremely convenient to use in modelling multicomponent solutions. However, spinodal calculations reveal that their calibration of this model for CaO-Al₂O₃-SiO₂ produces liquation tielines that do not appear to be in agreement with experimental work. In addition, their calibration contains some strongly negative excess entropy parameters and these permit a most unusual inverted liquation field to start at approximately >2115°C, wt% (SiO₂, Al₂O₃, CaO) = (70, 16, 14). This inverted field expands rapidly to cover most of the ternary for T> 2300°C and continues to expand at all higher temperatures. The Berman and Brown calibration for this system carries these negative excess entropies of mixing because the solution model is very strongly asymmetric as a result of the use of normal oxide mole weights in modelling the configurational entropy of mixing. A suggestion is made for a fairly natural restriction on the relative sizes of empirical models for excess versus configurational entropy.

Expressions are presented for the general consolute condition (all solution models) and for the second and third partials of the B&B G^x model.  相似文献   

Meltwater and the global ocean conveyor: northern versus southern connections     

Dan Seidov  Eric Barron  Bernd J. Haupt 《Global and Planetary Change》2001,30(3-4)

The sensitivity of the ocean circulation to changes in North Atlantic surface fluxes has become a major factor in explaining climate variability. The role of the Antarctic Bottom Water in modulating this variability has received much less attention, limiting the development of a complete understanding of decadal to millennial time-scale climate change. New analyses indicate that the southern deepwater source may change dramatically (e.g., experience a decrease of as much as two thirds during last 800 years). Such change can substantially alter the ocean circulation patterns of the last millennium. Additional analyses indicate that the Southern Hemisphere led the Northern Hemisphere changes in some of the glacial cycles of Pleistocene, implying a seesaw-type oscillation of the global ocean conveyor. The potential for melting of sea ice and ice sheets in the Antarctica associated with global warming can cause a further slowdown of the southern deepwater source. These results demand an assessment of the role of the Southern Ocean in driving changes of the global ocean circulation and climate. Systematic model simulation targeting the ocean circulation response to changes in surface salinity in the high latitudes of both Northern and Southern Hemispheres demonstrate that meltwater impacts in one hemisphere may lead to a strengthening of the thermohaline conveyor driven by the source in the opposite hemisphere. This, in turn, leads to significant changes in poleward heat transport. Further, meltwater events can lead to deep-sea warming and thermal expansion of abyssal water, that in turn cause a substantial sea-level change even without a major ice sheet melting.  相似文献   

Potential for photoenhanced toxicity of spilled oil in Prince William Sound and Gulf of Alaska waters     

Barron MG  Ka'aihue L 《Marine pollution bulletin》2001,43(1-6):86-92

Photoenhanced toxicity is the increase in the toxicity of a chemical in the presence of ultraviolet light (UV) compared to a standard laboratory test conducted with fluorescent lighting (minimal UV). Oil products, weathered oil, and specific polycyclic aromatic compounds present in oil are 2 to greater than 1000 times more toxic in the presence of UV. The photoenhanced toxicity of oil to fish and aquatic invertebrates appears to occur through a process of photosensitization, rather than photomodification of the aqueous phase oil. In photosensitization, the bioaccumulated chemical transfers light energy to other molecules causing toxicity through tissue damage rather than a narcosis mechanism. The available evidence indicates that phototoxic components of oil are specific 3-5 ring polycyclic aromatic hydrocarbons (PAHs) and heterocycles. Determinants of photoenhanced toxicity include the extent of oil bioaccumulation in aquatic organisms and the spectra and intensity of UV exposure. No studies have specifically investigated the photoenhanced toxicity of spilled oil in Alaska waters. Although there are substantial uncertainties, the results of this evaluation indicate there is potential for photoenhanced toxicity of spilled oil in Prince William Sound and the Gulf of Alaska. The potential hazard of photoenhanced toxicity may be greatest for embryo and larval stages of aquatic organisms that are relatively translucent to UV and inhabit the photic zone of the water column and intertidal areas. Photoenhanced toxicity should be considered in oil spill response because the spatial and temporal extent of injury to aquatic organisms may be underestimated if based on standard laboratory bioassays and existing toxicity databases. Additionally, the choice of counter measures and oil removal operations may influence the degree of photoenhanced toxicity.  相似文献   

Establishing irrigation potential of a hillside aquifer in the African highlands     

Seifu A. Tilahun  Debebe L. Yilak  Petra Schmitter  Fasikaw A. Zimale  Simon Langan  Jennie Barron  Jean-Yves Parlange  Tammo S. Steenhuis 《水文研究》2020,34(8):1741-1753

Feeding 9 billion people in 2050 will require sustainable development of all water resources, both surface and subsurface. Yet, little is known about the irrigation potential of hillside shallow aquifers in many highland settings in sub-Saharan Africa that are being considered for providing irrigation water during the dry monsoon phase for smallholder farmers. Information on the shallow groundwater being available in space and time on sloping lands might aid in increasing food production in the dry monsoon phase. Therefore, the research objective of this work is to estimate potential groundwater storage as a potential source of irrigation water for hillside aquifers where lateral subsurface flow is dominant. The research was carried out in the Robit Bata experimental watershed in the Lake Tana basin which is typical of many undulating watersheds in the Ethiopian highlands. Farmers have excavated more than 300 hand dug wells for irrigation. We used 42 of these wells to monitor water table fluctuation from April 16, 2014 to December 2015. Precipitation and runoff data were recorded for the same period. The temporal groundwater storage was estimated using two methods: one based on the water balance with rainfall as input and baseflow and evaporative losses leaving the watershed as outputs; the second based on the observed rise and fall of water levels in wells. We found that maximum groundwater storage was at the end of the rain phase in September after which it decreased linearly until the middle of December due to short groundwater retention times. In the remaining part of the dry season period, only wells located close to faults contained water. Thus, without additional water sources, sloping lands can only be used for significant irrigation inputs during the first 3 months out of the 8 months long dry season.  相似文献